This invention relates to spectroscopes which are used in different kinds of apparatus for spectral analyses such as emission and absorption spectral analyses. In particular, this invention relates to echelle spectroscopes which make use of an echelle diffraction grating.
Echelle diffraction gratings for use as a light-dispersing element for an echelle spectroscope are conventionally designed so as to have a larger blaze angle .theta. than ordinary echelle diffraction gratings, their free spectral range being shorter on the short-wavelength side and longer on the long-wavelength side so as to provide high dispersion and high resolving power. On the other hand, it has been known to be advantageous to use an image detector comprising a CCD or an array of photo-diodes as a light detector for detecting spectral light dispersed by a diffraction grating because the spectral light of each wavelength is detected in units of pixels and hence the structure of the light detector can be made simpler than if a conventional photo-multiplier or the like is used for the detection of light.
FIG. 6 shows an example of conventional echelle spectroscope comprised as a combination of an echelle diffraction grating and an image detector having such characteristics according to the so-called Czerny-Turner type arrangement. Light from a source 1 is passed through an entrance slit 2 and is directed to an echelle diffraction grating 4 after it is made into a parallel beam by means of a collimation mirror 3. Since the dispersed light from the echelle diffraction grating 4 includes overlapping among spectral light portions of different orders, it is further dispersed by a separating element 7 into these spectral light portions of different orders in a direction perpendicular to the dispersion direction by the echelle diffraction grating 4. The separated spectral light portions thus obtained are then reflected by an imaging mirror 9 such that they form images on an image detector 10. An ordinary echellette grating or prism may be used as the aforementioned separating element 7.
With the image detector 10 in an arrangement as shown in FIG. 6, however, it is difficult, as a practical matter, to increase the length of its light-receiving surface due to technical limitations on the production of semiconductor elements. As a result, a high resolving power could not be obtained if a measurement was to be made over a wide range of wavelengths. It may be attempted to produce an element with a long light-receiving surface but such an element would be very expensive and, since the aberration would be large accordingly, it would not be a practical solution.
In view of the above, there have been attempts to provide means for spectral analyses over a wide range of wavelengths with a high resolving power. U.S. Pat. No. 4,820,048, for example, disclosed an arrangement of a plurality of image detectors in order to cover wavelengths over a wide range. Another attempt was to provide a mechanism for shifting an image detector in several steps in the direction of wavelengths of the spectral light. Each of these attempts, however, has its own problems. If a plurality of image detectors must be used, for example, the total cost of the apparatus becomes high. If the image detector must be shifted from one position to another, the mechanism for effecting the shift is required to have the accuracy on the order of less than several .mu.m both in the horizontal and vertical directions in order to maintain reproducibility of the position of the spectral images. This again increases the overall cost of the apparatus. Since the shifting will be by mechanical means, furthermore, there will also arise the problem of stability. If an image detector with insufficient positional accuracy is shifted back and forth in the direction of the wavelength, the result of the measurements may in reality be as shown in FIGS. 7A, 7B and 7C due, for example, to a backlash, the actual measurements having been taken at slightly different wavelengths (with errors from the original wavelength .lambda.i shown by .increment..lambda. and .increment..lambda.') and thereby obtaining different measured intensities (with errors from the originally measured intensity Ii shown by .increment.I and .increment.I').